Nitrogen oxides, mainly NO and NO.sub.2, commonly referred to as NO.sub.x, are emitted from most combustion processes, such as automobile engines or fossil fuel power plants. Although the atmospheric chemistry of NO.sub.x species is complex and not well understood, many researchers believe that NO.sub.x species contribute to ecosystem damage and possibly human health problems through acid rain and tropospheric ozone formation. Additionally, NO.sub.x emissions and NO.sub.x abatement processes may be the primary source for the measured increase in ambient N.sub.2 O. N.sub.2 O contributes to global warming, or the greenhouse effect, as well as to stratospheric ozone depletion.
Relatively widespread and inexpensive approaches for controlling NO.sub.x emissions from combustion processes employ combustion modifications. However, combustion modifications, such as changes in operating conditions or low NO.sub.x burner designs, typically lead to NO.sub.x reductions of no more than 60%.
Post combustion, exhaust gas treatment processes are available which can achieve up to 90% NO.sub.x removal. Commercially available processes include selective catalytic reduction (SCR), the copper oxide system or Shell flue gas treatment (SFGT) process, and selective non-catalytic reduction (SNR). SNR with ammonia or urea operates in a relatively narrow range of high temperatures (850.degree.-1150.degree. C.). SFGT requires high levels of SO.sub.2 in the flue gas to generate the copper catalyst. Neither of these processes is appropriate for treating low sulphur exhaust gases from sources such as incinerators. High temperature processing is difficult in utility plants since it requires additive addition and mixing in a region of very rapid cooling, limiting the time available in the narrow temperature window.
SCR processes involve reacting ammonia (NH.sub.3) with NO.sub.x in a flue gas in the presence of a catalyst to form nitrogen (N.sub.2) and water (H.sub.2 O) Without a catalyst ammonia does not react at temperatures below about 850.degree. or 900.degree. C. In particular, NO does not react with NH.sub.3 at temperatures as low as approximately 400.degree. C. without a catalyst. SCR processes are expensive because catalyst lifetimes are limited, leading to high costs for catalyst replacement. In addition, even with a catalyst, the minimum temperature of NH.sub.3 -based processes is limited by undesirable deposition of NH.sub.4 HSO.sub.3 downstream.